Safety system for portable electrical apparatus



L. STORCH 2,554,593

SAFETY SYSTEM FOR PORTABLE ELECTRICAL APPARATUS May 29, 1951 Filed Oct. 8, 1946 uvvewron L; S ZOfiC H er a I N OE A TTOR/VE) Patented May 29, 1951 SAFETY SYSTEM FOR PORTABLE ELECTRICAL APPARATUS Leo Storch, Jersey City, N. J assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application October 8, 1946, Serial No. 702,045

1 Claim.

This invention relates to safety systems for electric apparatus. The object of the invention is to insure that the exposed metal parts of such apparatus cannot rise to dangerous potentials in the event that a defect develops in the apparatus.

It is, of course, common practice to ground the chassis and other exposed metal parts of electrical apparatus to keep them at ground potential. However, such ground connections may be broken or may develop such high resistance as to permit the protected parts to rise above ground potential when there is an insulation breakdown within the apparatus. This is particularly likely to occur when the apparatus is of the portable type since the ground lead ordinarily is less well protected than in permanent installations. When such portable apparatus is of a type which op crates at potentials of several thousand volts or more, special precautions to guard the operator are highly desirable.

According to the general features of this invention, the chassis and all other exposed metal parts of such apparatus are connected to ground through contacts on a relay which controls the application of power to the set. In order to insure that this connection is preserved, the relay is operated by a non-locking key and maintained operated by a holding connection shunting the key and extending through other contacts on the relay to the grounded chassis at a point other than the point of attachment of the ground lead. With this system, as will be more fully described, any electrical defect occurring in the system, which would create a hazard to the operator, will cause the immediate disconnection of the power supply from the set.

Fig. 1 of the drawing illustrates the invention as applied to a portable insulation breakdown test set of a well-known type and Fig. 2 shows a balanced source of power suitable for energizing the apparatus of Fig. 1.

In the drawing the chassis I is mounted on wheels 2 so that the set may be readily moved to the work location. Power is supplied from a conventional alternating current source 3 through a cord 4 and a circuit breaker 5 to the test apparatus, generally indicated at 6. This apparatus, for example, may be of any well-known type used for producing voltages and currents suitable for testing electric cables and in itself forms no part of the invention. While the voltage of the source 3 ordinarily will be that of the locally available power line, such as 440 volts between conductors I and 8 and about 250 volts from each of these conductors to ground at 9, the test apparatus 6 may be assumed to include transformers for producing higher test voltages which would be fatal to an operator in the event of an insulation breakdown within the apparatus.

When the set is to be used, the cord 4 is plugged into any convenient outlet I0 having a permanently grounded terminal II so that the line leads I and 8 are extended to the bus bars I2 and I3, respectively, and the conductor I4 is connected to the permanent ground. To condition the set for use the key I5 is operated momentarily to complete the operating circuit of relay I6 from the conductor I3 through the relay winding and the key to grounded conductor I4. If for any reason conductor I4 is not properly grounded, obviously the relay will not operate. Upon operation, the relay closes contact I! to complete a circuit for the operating coil ll! of the circuit breaker 5 which operates to apply working voltage to the apparatus 6. The relay also closes contacts I9 and 20 to complete a holding circuit for the relay extending from the lower end of the relay winding through the contacts I9, conductor 2I, a portion of the metal chassis I of the set, conductor 22 and contacts 20 to the grounded conductor I4. However, if either of the conductors H or 22 is not conductively connected to the chassis, the holding circuit will be incomplete and the relay will release upon the release of key I5. If the relay remains operated, the chassis is necessarily connected to ground I I and therefore cannot rise above ground potential even though failure of insulation should occur anywhere within the set.

It is important that the conductors 2I and 22 be attached to the chassis at separate points as indicated so that a portion of the chassis structure forms a part of the holding circuit. If this is not done, and there is a direct path between these conductors, the holding circuit will be complete even though a high resistance or an open circuit develops between these conductors and the chassis structure.

During the operation of the set in the normal manner, if the ground connection to the chassis should fail at any point, the holding circuit of the relay I6 is broken and the relay releases and releases the circuit breaker 5 to de-energize the high voltage apparatus 6.

The invention has been described with reference to the protection of a portable test set, but it is equally applicable to any other type of electrical apparatus, portable or otherwise, where a ground connection is important. The circuit shown is energized by line conductors in balanced relation to ground as in a three phase, star-connected system, but obviously it may be used with an unbalanced source where, for example, bus bar I2 is at ground potential, as in ordinary lighting circuits. In that case conductor l4 would be connected to conductor I2 and the cord 4 would be of the two conductor type.

These and various other modifications may be made within the scope of the following claim.

What is claimed is:

In a safety system for portable electrical apparatus having a metal part normally insulated from ground, a source of power including one grounded and at least one ungrounded lead, a

circuit breaker for connecting the ungrounded 15 circuit breaker operating coil circuit, and a relay holding connection in parallel with the key which includes the other sets of relay contacts, a portion of the normally insulated metal part of the apparatus, and leads connecting this portion of the normally insulated metal part to the other sets of relay contacts.

LEO STORCH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,381,302 Harvey et al June 14, 1921 2,207,202 La Deau July 9, 1940 FOREIGN PATENTS 20 Number Country Date 350,751 Great Britain June 18, 1931 427,183 Great Britain Apr. 17, 1935 487,935 Great Britain June 27, 1938 

